With the aid of shoe soles, shoes are provided with a wealth of different characteristics that may be pronounced in various strengths depending on the specific type of shoe. Primarily, the shoe soles have a protective function. They protect the foot of the respective wearer by way of their increased stiffness in comparison with the shoe shaft from injuries through e.g. sharp objects on which the shoe wearer treads. Furthermore, the outsole typically protects the midsole of the shoe from excessive wear by an increased abrasion resistance. It can also be a function of a shoe sole to provide a certain stability. Additionally, a shoe sole can provide a cushioning effect, for example to cushion or absorb the forces occurring during contact of the shoe with the ground. Furthermore, a shoe sole can protect the foot from dirt or spray water.
In order to meet this wealth of functionalities, different materials are known from the prior art from which shoe soles or individual parts of such soles may be made. As examples, shoe soles or parts of shoe soles made from ethylene-vinyl-acetate (EVA), thermoplastic polyurethane (TPU), particle foam out of expanded thermoplastic urethane (eTPU) or expanded polypropylene (ePP), rubber, polypropylene (PP) or polystyrene shall be mentioned here. Each of these different materials provides a specific combination of different properties which are, depending on the respective requirement profiles, more or less well suited for the soles of particular shoe types.
Therefore, the use of expanded materials, in particular the use of particle foam from expanded thermoplastic urethane (eTPU), has been considered for the construction of shoe soles, for example in WO 2005/066250 A1.
A further function of shoe soles may be to increase the adhesion or grip of a shoe on the respective ground in order to facilitate a faster movement and to minimize the risk of a fall of the wearer. To this end, the outsole of a shoe can, for example, be provided with a profile and the shoe can have a number of knobs, cleats, spikes and the like.
For example, U.S. Pat. No. 4,085,527 describes an athletic shoe having a sole which includes a cushioning pad and a plurality of cleats extending from the lower surface of the sole and being particularly configured in the heel region to provide stability and effective cushioning during running. However, a disadvantage of this construction is, in particular, that the cushioning pad extends in a planar and roughly evenly thick manner throughout the entire sole and therefore influences the properties of the sole across the entire sole area in the same manner, without a possibility to selectively control the cushioning- and stability characteristics. This can, in particular, result in the sole not having the desired stability in the midfoot area.
A sole construction is furthermore known from WO 03/071893 A1, which comprises in some embodiments a spring member that includes at least one primary stud and one or more secondary studs, which operate to engage with the ground when an impact force causes the primary stud to deform towards the sole. In this way, additional grip may be provided on varying surface conditions. This construction is, however, technically very complex and is mainly suited for shoes with cleats and/or knobs, e.g. football shoes.
Other sole constructions, in particular for shoes with cleats or knobs, are described in U.S. Pat. No. 6,145,221, as well as in WO 98/08405 A1.
A general disadvantage of the shoe soles known from the prior art is that they are typically intended for a specific type of use only, or are tuned to particular surfaces/ground conditions. Thus, for example, shoes with cleats are particularly well suited for use on soft ground, for example a grass pitch, whereas shoes with spikes can mainly be used on a tartan track, a golf course, or the like. Both types of shoes are, however, not well suited for running on hard ground, as for example asphalt. Other shoes, as for example indoor soccer shoes or basketball shoes, typically have a sole with only a mild profile in order to ensure as large a contact area with the indoor surface as possible. Such shoes do not, however, provide sufficient grip on, for example, (wet) grass or moist forest soil. Especially in the area of running sports, in particular during jogging or fitness runs, a wearer is often confronted with different ground- and surface conditions. For example, a runner can initially cover a part of a run on a pavement or a street and then change to a soft forest trail.
Starting from prior art, it is therefore the underlying problem of the present invention to provide a shoe sole which is capable of adapting to such varying conditions and in particular to provide as good a grip on different grounds as possible. At the same time, the manufacturing expenses shall be as small as possible.